1. Your house plants are alive, and you can't smoke any of them.

Only one plant, and it's a yucca. No care required, see :)
2. Having sex in a twin bed is out of the question.

I'm not there yet.
3. You keep more food than beer in the fridge.

Always have done. I'm not much of a one for drinking at home and never have been.
4. 6:00 AM is when you get up, not when you go to bed.

Yep. Only in my case, it's 0530.
5. You hear you favorite song on an elevator.

I take the stairs, you insensitive clod.
6. You watch the Weather Channel.

I live in the UK, you insensitive clod.
7. Your friends marry and divorce instead of hook up and break up.

They're all just getting married. Give it 5 years or so.
8. You go from 130 days of vacation time to 14.

25. And rising.
9. Jeans and a sweater no longer qualify as "dressed up".

Sure it does, if the jeans are clean :-)
10. You're the one calling the police because those damn kids next door won't turn down the stereo.

Nah.
11. Older relatives feel comfortable telling sex jokes around you.

I'm running out of those :\\
12. You don't know what time Taco Bell or White Castle closes anymore.

I don't know what they ARE, other than they're lardyboy nosheries in the US.
13. Your car insurance goes down and your payments go up.

Half right. The first half.
14. You feed your dog Science Diet instead of McDonalds leftovers.

Cat gets fed better than I do :\\
15. Sleeping on the couch makes your back hurt.

Check.
16. You no longer take naps from noon to 6 PM.

Check.
17. Dinner and a movie is the whole date instead of the beginning of one.

Date? What ees thees theeng you earth people call "date"?
18. Eating a basket of chicken wings at 3 AM would severely upset, rather than settle your stomach.

Nah. I'm rather fortunate with my constutition (and general health; while unfit, I'm not prone to illness)
19. You go to the drug store for ibuprofen and antacid, not condoms and pregnancy tests.

s/and antacid, //
20. A $4.00 bottle of wine is no longer "pretty good stuff".

It all tastes the same to me. But then I'm a philistine (cf. my "all whiskey is paintstripper" thesis)
21. You actually eat breakfast food at breakfast time.

Everyone knows that left-over chicken dhansak with keema pilau is the cornerstone of any nutritious breakfast.
22. "I just can't drink the way I used to," replaces, "I'm never going to drink that much again."

Heh. No comment.
23. 90% of the time you spend in front of a computer is for real work.

Actually, the amount of computer time that's work-related is decreasing; I spend more time dealing with people these days. Gotta be a plus, no?
24. You drink at home to save money before going to a bar.

See earlier.
25. You read this entire list looking desperately for one sign that doesn't apply to you and can't find one to save your sorry old ass.

It's preventative with me, buddy.